This invention pertains to display systems and more particularly to such display systems utilizing raster scans.
In the many areas there is a need for systems which display text. Of the many areas, computer terminals and word processors rely heavily on visual displays using cathode-ray tubes. In cathode ray tube (CRT) displays, the electron beam is moved to different points of the screen to paint the graphics of the text. There are three general techniques for generating the characters. In one technique, there is a stencil within the tube and the electron beam is first directed to the position in the stencil having the desired graphics and after the beam is shaped by the opening it is directed to the desired position on the screen. Such devices as represented by the Charactron have very expensive electron deflecting structures and have limited font capacity. The second technique utilized vector or segment generators and is exemplified by U.S. Pat. Nos. 3,579,023 and 3,335,416. Such generators require complicated electronics using stabilized analog operational amplifiers. The third technique is borrowed from conventional television receivers and is ideally suited for inexpensive mass produced displays.
The third technique relies on the generation of a scanning raster comprising a plurality of parallel sweeps of the beam across the screen. During the sweeps, the beam is intensity modulated to form the graphics. One variation as exemplified in U.S. Pat. No. 3,471,848 generates a raster of vertical lines and the graphic is "painted" by means of strokes. This variation is popular when high quality characters are desired. Another variation as exemplified in U.S. Pat. No. 3,803,583 which generates a raster of horizontal lines and the graphic is "painted" by dots. This second variation has been very popular for relatively low cost displays used in word processors and computer terminals having CRT monitors.
In CRT monitors utilizing dot matrices and raster scans, the line scanning can be performed using either sequential or interlaced techniques. For given horizontal and vertical sweep rates twice the total number of graphics can be displayed on the CRT screen using interlaced scanning rather than sequential scanning. With vertical sweep rates of 60 Hz or 50 Hz, flicker, or interline scintillation, is present with interlaced scanning whereas no flicker is noticeable with sequential scanning. For this reason sequential scanning is preferred except where the quantity of data to be displayed forces the designer to employ interlaced scanning.
In the field of CRT terminals the trend is toward displaying larger quantities of alphameric data. Consequently, the application of interlaced scanning and higher resolution CRT monitors are increasing together with the use of longer persistance CRT phosphors in order to display the increased quantity of data without the annoyance of flicker.
Previously, a 2,000 character display was considered large. Currently, 4,000 to 8,000 character displays are becoming common. In order to keep the size of the CRT terminal within acceptable limits for office environments, this increased volume of data is often displayed on relatively small CRT screen sizes. Typically, 15-inch diagonal screens are employed allowing usable display dimensions approximating an 81/2 inches .times. 111/2 inches sheet of paper. Displaying from 4 to 8 thousand characters within these dimensions results in relatively small characters. Despite the finer resolution and freedom from flicker provided by the better-engineered displays, experience with such displays are beginning to indicate that operators tire more readily due to prolonged viewing of the smaller characters.